The present invention relates generally to reducing friction forces between components in an internal combustion engine, and more particularly to using a solid lubricant as an additive to an engine oil to form a low friction surface between the cylinder wall or bore and one or more of the piston and piston rings.
The cylinder walls and cylinder liners of an internal combustion engine (ICE) are manufactured to exacting standards with close surface roughness tolerances as a way to keep friction wear to a minimum. Liquid lubricants such as engine oil (also referred to herein as motor oil) are used to further reduce friction, as well as a way to transfer heat generated in the cylinder during engine operation. Typically, the lubricant occupies the space provided by the residual roughness in the cylinder or liner walls to help provide a film-like layer between the cylinder and the reciprocating piston and piston ring assembly. Nevertheless, liquid lubricants have certain limitations. For example, in situations where friction-reducing additives are used, the low solubility of such additives in conventional lubricant base fluids reduces the benefit of such additives.
Other forms of lubricants, such as solid lubricants applied to or otherwise formed on one or more of the interacting component surfaces, may be used to further improve wear-reduction properties. US Published Patent Application 2010/0272931—which is incorporated by reference in its entirety—discloses a method of tribochemically depositing (i.e., by friction stimulating) a solid lubricant substance directly onto the rough curved surface of an engine cylinder wall or liner wall using an oxide, carbide or silicide tool with motion in at least two transverse directions such that a stable sulfide is formed to improve the friction properties of the wall, particularly in the mixed and boundary lubrication regimes, where actual contact between the moving surfaces is most likely to occur. Examples of such solid lubricants may include graphite, molybdenum disulfide (MoS2) and tungsten disulfide (WS2), all of which exhibit desirable friction properties. It is generally perceived that tribochemical approaches have the ability to produce very smooth, defect-free surfaces.
Nevertheless, it is difficult and expensive to provide solid lubricant surface treatment for complex component shapes. This is especially true for cylinder walls, where limited spaces coupled with inherent non-planarity make it difficult to achieve with thermal decomposition, physical vapor deposition (PVD), chemical vapor deposition (CVD) electroplating, spraying or related known solid lubricant deposition techniques. In addition, the metallic makeup of the part being coated may exacerbate the solid lubricant deposition problem.